In recent years, telephone communication systems have expanded from traditional Plain Old Telephone System (POTS) communications to include high-speed data communications as well. As is known, POTS communications include the transmission of voice information, Public Switched Telephone Network (PSTN) control signals, as well as, information from ancillary equipment in analog form (i.e., computer modems and facsimile machines) that is transmitted in the POTS bandwidth.
Prompted largely by the desire of large businesses to reliably transfer information over a broadband network, telecommunications service providers have employed Discrete Multi-Tone (DMT) systems to provide a plethora of interactive multi-media digital signals over the same existing POTS twisted-pair lines. The practice of installing Asymmetric Digital Subscriber Lines (ADSL) using DMT systems to communicate both voice and data signals to a customer premise from a central office has proliferated over recent years. Since ADSL signals are transmitted in a higher frequency band than that of the POTS frequency band, transmitting signals from both the POTS and ADSL frequency bands over the same twisted-pair telephone line (even at the same time), is possible. Specifically, the POTS frequency band is generally defined from 0 Hz to 4 kHz, while ADSL frequency band is defined by a lower cutoff frequency of approximately 25 kHz and an upper cutoff frequency of approximately 1 MHz.
The key to ADSL is that the upstream and downstream bandwidths are asymmetric, or uneven. In practice, the bandwidth from the provider to the user (i.e., the downstream channel) will be the higher speed path. This is due to the limitation of the telephone cabling system and the desire to accommodate the typical Internet and video on demand usage patterns. The majority of data being transferred in support of both Internet and video on demand services is being sent to the user (e.g., computer programs, graphics, sounds and video) with minimal upload capacity required (e.g., keystrokes and mouse clicks). Downstream speeds typically range from 1.5 Mbps to 9 Mbps. Upstream speeds typically range from 64 kbps to 1.5 Mbps.
Reference is made to FIG. 1, which illustrates the delivery of broadband communication services via a prior art ADSL communication system 1 over a twisted-pair telephone line. In this regard, a central office 10 is configured to provide broadband services which it assembles via central office ADSL modems 45 for transmission over a twisted-pair telephone line 48 to a customer premise 50, or in the alternative, another central office 10. Examples of such broadband services are depicted as video conferencing 15, Internet 20, telephone services 25, movies on demand 30, and broadcast media 35. The central office 10 may assemble signals from the aforementioned broadband services via a Digital Subscriber Line Access Multiplexer (DSLAM) 40 for appropriate transformation and transmission by a plurality of ADSL modems 45. Each of the ADSL modems 45 may be in communication via a dedicated twisted-pair telephone line 48 with a suitably configured ADSL modem 55 at a customer premise 50.
As illustrated in FIG. 1, the DSLAM 40 and each of a plurality of ADSL modems 45 may be assembled within an ADSL service rack 36 within the central office 10. For simplicity of illustration and explanation, the ADSL communication system 1 presented in FIG. 1 is shown with a single ADSL service rack 36 for communicating each of the broadband services to N ADSL modems 55. The ADSL service rack 36 may be configured to supply conditioned resources necessary to support the operation of the N ADSL modems 45. Those skilled in the art will appreciate the scalability of the ADSL communication system 1 generally presented in FIG. 1. For example, the central office 10 may be configured with a plurality of Transmission Control Protocol/Internet Protocol (TCP/IP) routers and Asynchronous Transfer Mode (ATM) switches (not shown) that may distribute one or more broadband service signals to a plurality of DSLAMs 40. In turn, the plurality of DSLAMs 40 may further distribute the broadband service signals to a plurality of remotely located ADSL modems 55.
At the opposite end of the twisted-pair telephone line 48, the customer premise 50 may be configured with a compatible ADSL modem 55, which may be configured to process and distribute the multiple broadband services to appropriate destination devices such as a computer, television, and a digital telephone as illustrated. It is significant to note that that the customer premise 50 may have POTS devices such as a facsimile machine and an analog (POTS) telephone integrated on the twisted-pair telephone line 48 along with the ADSL modem 55. It is also feasible that the customer premise 50 may be replaced in some applications by another central office 10 or an ADSL repeater, where the POTS service may not be available or needed.
In the United States, an ADSL standard for physical layer operation was first described in the American National Standards Institute (ANSI) T1.413-1995, the Network and Customer Installation Interfaces—Asymmetric Digital Subscriber Line (ADSL) Metallic Interface specification. This document describes how ADSL equipment is to communicate over a previously analog local subscriber loop of the PSTN. The document does not describe the entire ADSL network architecture and services, or the internal functioning of the ADSL access node. The T1.413-1995 standard specifies such fundamentals as line coding (how the bits are sent) and the frame structure (how the bits are organized) on the twisted-pair conductor comprising the local subscriber loop. The T1.413-1995 standard specifies that compliant ADSL transceivers must use discrete multi-tone (DMT) coding with either frequency division multiplexing (FDM) or echo cancellation to achieve full-duplex operation.
DMT divides the entire bandwidth range on the formerly analog pass band of the local subscriber loop into a large number of equally spaced channels or bins. Technically, they are sub-carriers, but many call them sub-channels. The entire bandwidth of approximately 1.1 MHz is divided into 256 sub-channels, starting at 0 Hz. Each sub-channel occupies 4.3125 kHz. Some of the sub-channels are reserved for special signals, while others are not used at all. For example, channel #64 at 276 kHz is reserved for a pilot signal. The general concept behind DMT and its use with regard to ADSL communication systems is illustrated in FIGS. 2 through 4.
Looking at FIG. 2, the plot generally denoted by reference numeral 60 shows the number of encoded bits per channel per second that may be transferred (i.e., transmitted across) an ideal communication system. As illustrated in the plot 60 of FIG. 2, the number of bits transferred over time across each of the 256 sub-channels in an ideal (i.e., loss less) communication system would be constant and limited only by the transmit power capacity of the transmitter.
However, a number of environmental factors affect the maximum throughput of an ADSL communication system 1 (FIG. 1) as illustrated in FIG. 3. First, the signal gain (i.e., the reciprocal of attenuation or system signal loss) varies as a function of frequency. At higher frequencies, distance effects dominate; at lower frequencies, impulse noise and crosstalk effects dominate. This leaves the broad middle range from about 25 kHz to 1.1 MHz for signals, with the gain slowly dropping off with increasing frequency. Second, bridged taps may affect the frequency response of individual twisted-pair telephone transmission lines 48 (FIG. 1) as evidenced by the notch in the middle of the real loop gain plot 70 of FIG. 3. Last, the bandwidth from 25 kHz to 1.1 MHz is shared with radio frequency broadcast transmitters. Local broadcasts originating at one or more Amplitude Modulation (AM) radio stations may interfere with several DMT sub-channels rendering the ADSL signal to noise ratio too low for error free data transfers over the affected sub-channels.
As a result, most DMT systems use only 250 or less sub-channels for information transfers. The lower sub-channels, #1 through #6 in most cases, are reserved for the 0 Hz to 4 kHz pass band for analog voice. The signal loss at the upper channels, such as #250 and above, as illustrated by the actual loop gain plot 70 of FIG. 3 is usually so great that it is difficult to use them for information transfer across long subscriber loops at all.
A typical DMT sub-channel distribution is presented in FIG. 4. The exemplary bits per channel plot 80 shows that the lower frequency sub-channels may be reserved for analog voice or POTS communications. Next, there are 32 upstream channels, usually starting at channel #7. Last, the remaining sub-channels may be used for downstream data transmissions, which gives ADSL its characteristic asymmetric bandwidth. When echo cancellation is used, the upstream and downstream frequency ranges may overlap providing approximately 250 sub-channels for downstream data transfers. When FDM is used for echo control, as illustrated in the plot 80 of FIG. 4, usually only 218 downstream channels remain available for downstream data transfers as a number of the sub-channels are removed to form a guard band between the upstream and the downstream channels.
As further illustrated in FIG. 4, the upstream channels occupy the lower frequency channels whereas the downstream channels occupy the higher frequency channels. The upstream channels occupy the lower end of the spectrum for two reasons. First, the signal attenuation is less over that range of frequencies and customer transceivers are typically lower-powered than local exchange transceivers. Second, there is more noise at the local exchange transceivers (i.e., within the central office 10) and a higher probability of crosstalk, so it makes sense to use the lower frequencies for upstream signal transmissions.
When ADSL transceivers that employ DMT are activated, each of the sub-channels is tested by the transceivers. The testing procedure is a complex handshaking procedure whereby the transceivers determine an appropriate gain (the reciprocal of attenuation) and noise for each of the sub-channels. Usually, each of the numerous sub-channels available for data transfers employs a quadrature amplitude modulation (QAM) coding technique to send bits over the sub-channel. The total system throughput is the sum of all the QAM bits sent via the active sub-channels.
Moreover, all active sub-channels are constantly being monitored for performance and errors. The data transfer rate attributable to each individual sub-channel or group of sub-channels can actually vary, giving DMT a granularity of 32 kbps. In other words, a DMT device (i.e., an ADSL transceiver) may operate at 768 kbps or 736 kbps (i.e., 32 kbps less) or at other rates which are multiples of 32 kpbs, depending on operational and environmental conditions.
DMT systems, by nature of their distribution across multiple frequency bands, are capable of retuning devices to optimize data transfer rates for changing line conditions. DMT devices selectively transfer bits from the data stream in those discrete frequency bands that are uncorrupted from AM radio interference and unaffected by phone system bridge taps, thereby tuning, or maximizing performance under changing line conditions.
The ANSI T1.413-1995 standard for ADSL communication over a PSTN provided twisted-pair telephone line 48 (FIG. 1) permits a great deal of flexibility in implementation details. While the T1.413 standard provides for data connectivity between multiple equipment manufacturers, the standard does not assure optimum performance between modems supplied by different manufacturers. Each manufacturer, using the flexibility provided by the T1.413 standard, typically generates algorithms that take into account the advantages and limitations of their particular hardware design to maximize system performance when communicating with their own modems. Moreover, there are many portions of the T1.413 standard that are interpreted differently by the various manufacturers. For example, the capability of DMT based ADSL systems to adaptively adjust the data transfer rate is designed to achieve a data transfer rate that is within 32 kbps of the highest achievable data transfer rate given the length of a particular twisted-pair telephone line 48 (FIG. 1) and present environmental variables (e.g., the ambient temperature, humidity, the presence of one or more bridged taps, and RF interference affecting the line). However, the achieved data rate between a pair of DMT based modems originating from two different manufacturers is highly dependent on the type of filters used by each modem, how well the equalization algorithms used in one modem work with the other, and various other DSP related parameters. There can also be subtle incompatibilities in the rate negotiation portion of the training phase. For at least these reasons the actual data rate achieved between a pair of modems from different manufacturers can be well below the maximum possible rate for the given line conditions.
According to the T1.413-1995 standard, in order to establish a data connection between two ADSL compliant modems, the modems must perform a startup procedure or training phase. During the training phase, in addition to measuring actual twisted-pair telephone line 48 (FIG. 1) conditions, the modems exchange pre-assigned manufacturer identifications (IDs). Even though the modems exchange vendor or manufacturer IDs during the training process, the exchange occurs after the early phases of the startup training where equalizers and filters critical to data transfer performance are configured.
In light of the expected implementation and operational cost erosion for all data interface technologies, it is highly desirable to identify and implement communication links that exhibit increased performance with minimal added cost and complexity. With this goal in mind, there is a need for an improved system and method that can provide optimum performance between ADSL transceivers produced by different manufacturers, while minimizing installation and operational complexity, space requirements, and cost.